System and method for cutting using a variable astigmatic focal beam spot

ABSTRACT

A variable astigmatic focal beam spot is formed using lasers with an anamorphic beam delivery system. The variable astigmatic focal beam spot can be used for cutting applications, for example, to scribe semiconductor wafers such as light emitting diode (LED) wafers. The exemplary anamorphic beam delivery system comprises a series of optical components, which deliberately introduce astigmatism to produce focal points separated into two principal meridians, i.e. vertical and horizontal. The astigmatic focal points result in an asymmetric, yet sharply focused, beam spot that consists of sharpened leading and trailing edges. Adjusting the astigmatic focal points changes the aspect ratio of the compressed focal beam spot, allowing adjustment of energy density at the target without affecting laser output power. Scribing wafers with properly optimized energy and power density increases scribing speeds while minimizing excessive heating and collateral material damage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of co-pending U.S. Provisional Patent Application Ser. No. 60/448,503, filed on Feb. 19, 2003, which is fully incorporated herein by reference.

TECHNICAL FIELD

This invention relates to laser cutting or machining, and more particularly, relates to a system and method for forming a variable astigmatic focal beam spot using solid-state lasers with an anamorphic beam delivery system to scribe semiconductor wafers.

BACKGROUND INFORMATION

Increasing worldwide demand for compound semiconductor devices has driven the development of high throughput, cost effective, and reliable production techniques and equipment. Compound semiconductors are comprised of a combination of group-III elements, such as B, Al, Ga, In and Ti, and group-V elements, such as N, P, As, Sb and Bi. A typical example of III-V compound semiconductor devices is light emitting diodes (LEDs) using InP, GaP, GaAs, AlInGaP and GaN.

Among theses LEDs, blue LEDs consist of multiple Gallium Nitride-based layers epitaxially grown on a silicon carbide or sapphire wafer substrate. Silicon carbide wafers have been diced using high-precision saws. Sapphire wafer die separation has been performed by mechanical scribing with a diamond tool. The wafer can then be cleaved along the scribed lines via a fracturing machine. The extreme hardness of blue LED substrates and the small LED die size create significant problems for both saw dicing and mechanical scribing, including low die yield, low throughput, and high operating costs. The brittle substrates, such as GaP and GaAs, also show low productivity due to excessive edge chipping by the mechanical scribe and break and the saw dicing processes. Moreover, the conventional processes require relatively large cutting areas, reducing the number of devices on a wafer.

Scribing with ultra violet (UV) lasers has emerged as an alternative method for separating compound semiconductor wafers. Excimer lasers and diode pumped solid-state (DPSS) lasers are two major light sources for UV laser scribing. When short duration UV laser light pulses are tightly focused onto the wafer surface, each pulse is absorbed into a sub-micron thick surface layer, which then vaporizes. The vaporized material carries away the energy of the interaction, minimizing heat transfer to the surrounding material. This process is known as photo-ablation. In order to produce deep cuts, hundreds of successive laser pulses are focused onto the surface.

Moving the wafer under a rapidly pulsed, focused laser beam produces an extremely narrow ‘V’ shaped cut, the depth of which is controlled by the scan speed. Typically, these cuts terminate 30-50% into the thickness of the wafer. After laser scribing, the wafer is fractured using standard cleaving equipment. The ‘V’ shaped laser cuts act as stress concentrators, inducing well-controlled fracturing with excellent die yield.

Efficient photo-ablation is preferred for laser scribing and depends strongly on two properties of the UV laser light: wavelength and pulse duration. In general, photo-ablation benefits from shorter laser wavelength and shorter pulse duration for both optical and thermal reasons. In the formula, E=h(c/λ), the photon energy, E, is inversely proportional to λ, the photon wavelength. Simply put, shorter wavelengths impart more energy per photon. The benefits achieved by short laser wavelengths include improved optical absorption, reduced absorption depth, lower irradiance required for ablation, and reduced cut width. In the formula, I=E/At, the irradiance, I, is proportional to pulse energy, E, and inversely proportional to both beam area, A, and pulse duration, t. As a result, short pulse durations result in higher irradiances for a given spot size and pulse energy. Also, short pulses deliver successful ablation at larger spot sizes on target, resulting in more rapid cutting. The benefits achieved by short laser pulse duration include increased irradiance on target, and reduced heat transfer to the substrate due to more rapid absorption and ablation.

In silicon carbide, optical wavelengths below 370 nm have photon energies that exceed the bandgap of the material, resulting in direct photon absorption. For example, the photonic energy of a DPSS 355 nm solid state laser beam (3.5 eV) is higher than the highest bandgap of silicon carbide (3.27 eV for 4H polytype), resulting in strong absorption followed by ablation. Sapphire, conversely, has a bandgap (9.9 eV) that is higher than the photon energy of commercially-available UV lasers, for example, an F₂ laser at 157 nm (7.9 eV). In such cases, multi-photon absorption can induce efficient optical absorption and the necessary irradiance (W/cm²) for multi-photon absorption can be very high. The efficiency of multi-photon absorption in sapphire is strongly wavelength-dependent. Shorter wavelengths are absorbed more completely in sapphire, resulting in less heat input to the bulk material. For photo-ablation to occur, light that is absorbed must impart sufficient energy to vaporize the material. The threshold irradiance for ablation is also strongly wavelength-dependent. The higher photon energy and smaller absorption depth of shorter optical wavelengths result in ablation at lower irradiance levels.

As compared to the sapphire, compound semiconductor substrates usually have lower bandgap energy, such as GaN (3.3 eV), GaP (2.26 eV) and GaAs (1.42 eV). Although coupling of a UV laser at 266 nm is efficient on these substrates, excessive photonic energy under this strong absorption can result in unnecessary thermal conduction to the substrates, causing heat related damages. In contrast, insufficient laser energy density can result in improper ablation, even with the strong absorption. Thus, an optimum laser energy density or irradiance is an important factor in laser scribing, which leads to a higher scribing speed and/or maximized productivity.

Among UV lasers, excimer lasers generate the most power output, e.g., nearly 100 watts in the deep UV range. These advantages have made the excimer laser uniquely suited for hard LED wafer scribing. Scribing using excimer laser technology has involved the delivery of a line beam or series of line beams onto an LED wafer, which is translated by a controlled motion stage to be diced in a desired direction. Excimer laser scribing, for example, using KrF 248 nm light output, utilizes a near-field imaging technique through which a patterned laser beam is projected from a mask. Thus, the delivery of a line beam has been achieved by line-patterned mask projection. One example of the use of excimer laser patterned projections is disclosed in U.S. Pat. No. 6,413,839, incorporated herein by reference. In this patent, a single line beam and multiple line beams are projected onto a sapphire wafer with blue LEDs.

When using a mask with excimer laser projection techniques, the modification of a patterned beam is relatively simple and is achieved by changing the shape of the mask. For example, the narrow line beam for scribing (i.e., usually in tens of microns) is projected by a slit mask. However, the slit mask transmits the laser beam only through an open area of the mask. Thus, introducing the mask in a beam delivery system (BDS) blocks a major portion of the laser beam, which makes the beam utilization factor (BUF) low. This low beam utilization factor limits the speed of the scribing process, because the scribing speed is mainly proportional to the size of the projected beam in the scribing translation direction.

Recently, developments in UV solid-state laser technology have resulted in DPSS lasers with sufficient average power to be considered for the scribing of hard compound semiconductor wafers, such as those made of sapphire and silicon carbide. A few laser manufacturers have developed the third harmonic (355 nm) and the fourth harmonic (266 nm) DPSS lasers with a gain medium for Nd³⁺ ions doped in a yttrium\crystalline matrix (Nd:YVO₄ or Nd:YAG). These frequency-multiplied DPSS lasers demonstrate significant improvements in pulse duration, frequency and power output. For example, UV solid-state lasers running at the third harmonic (355 nm) now achieve average powers of over 5 Watts, and the fourth harmonic (266 nm) lasers achieve average powers of over 2.5 Watts. Also, these lasers offer short pulse durations, e.g., below 20 nanoseconds. UV solid-state lasers with short wavelength and pulse duration (e.g., less than 15 nanoseconds) can create extremely high irradiance, e.g., over 10⁹ W/cm², resulting in instantaneous vaporization by photonic bombardment. This extreme irradiance of frequency-multiplied DPSS lasers makes the separation of the hard substrate possible. For an example, although the sapphire has a high optical transmissivity to UV wavelengths, this extreme temporal and spatial concentration of photons results in effective multi-photon absorption, causing ablation.

Generally, UV solid-state lasers generate a circular Gaussian beam in TEM₀₀ mode and current methods of UV solid-state laser scribing utilize a focused circular beam spot. Unlike an excimer laser BDS, DPSS lasers utilize far field imaging, which does not require patterned mask imaging. Examples of laser scribing using far field imaging are disclosed in U.S. Pat. No. 5,631,190 and U.S. Pat. No. 6,580,054, incorporated herein by reference. The raw beam from the laser resonator is directly focused by a beam-focusing lens and is delivered to the target. The BUF is higher because the BDS for a DPSS laser utilizes the full beam. The scribing speed is slower, however, due to the small size of the focused beam spot, which is one drawback of the application of the DPSS laser to mass-production. Also, the conventional beam delivery system used in a DPSS laser has a limited ability to control the adjustment of laser processing parameters. In the conventional scribing techniques using DPSS lasers, laser processing parameters are controlled by adjusting laser output power and the directed laser light is not modified.

In general, laser beams must be focused for laser material processing applications. A focused laser beam has two important characteristics; 1) optimum laser intensity (usually expressed by the laser energy density J/cm²) for a target material, and 2) minimum size of a focused spot or a beam waist diameter. The optimum laser intensity is important to achieving a desired processing result, because either excessive or insufficient laser intensity will introduce imperfections into the process. In addition, the focused beam spot should have sufficient flexibility to adjust its intensity, since the optimum intensity is determined by the light absorption properties of the particular target material. The minimum size of a beam waist diameter is important when laser material processing requires a sharply focused beam for fine resolution.

Another issue with laser scribing processes is caused by the ablation induced debris generated along the wake of the cut. The debris on the semiconductor dies or LED dies are detrimental to both their performance and packaging. Photoresists for lithography have been applied on substrate surfaces for the protective coating to prevent the debris, but the photoresist tends to be carbonized by the heat from the laser induced plasma. The carbonized photoresist is hard to remove, especially near the laser cuts. Adhesive tapes have also been proposed as protection, but related procedures, such as changing the adhesive tape after scribing in every single direction, are not conducive to mass production. In addition, excessive amounts of residue remained after the laser scribing because of the high thickness of the tape together with the adhesive.

Accordingly, there is a need for a laser scribing system and method that avoids the drawbacks of the existing techniques, is capable of using shorter wavelengths and pulse duration, and is capable of optimizing laser intensity and minimizing beam waist diameter.

SUMMARY

In accordance with one aspect of the present invention, a method is provided for forming a variable astigmatic focal beam spot to cut a substrate. The method comprises the steps of generating a raw laser beam, expanding the raw laser beam, and modifying the expanded beam such that the modified beam is collimated in one principal meridian and converging in another principal meridian. The modified beam is focused to produce an astigmatic focal beam spot having an elongated shape and two separate focal points directing the astigmatic focal beam spot at the substrate to obtain at least a partial cut in the substrate.

According to another aspect of the present invention, a method is provided for scribing a semiconductor wafer using a laser. According to this method, a laser beam is generated and an astigmatic focal beam spot is formed by modifying the laser beam such that the modified beam spot has two separate focal points and the astigmatic focal beam spot has an elongated shape. The astigmatic focal beam spot is directed at a surface of the semiconductor wafer. The astigmatic focal beam spot is applied with a set of parameters until one or more partial cuts are obtained in the semiconductor wafer.

In accordance with another aspect of the present invention, a method is provided for separating semiconductor wafers into dies. According to this method, a laser beam is generated and an astigmatic focal beam spot is formed by modifying the laser beam such that the modified beam spot has two separate focal points and the astigmatic focal beam spot has an elongated shape. The astigmatic beam spot is directed at a surface of a semiconductor wafer. The variable astigmatic focal beam spot is applied with a set of parameters until one or more partial cuts are obtained in the semiconductor wafer. The semiconductor wafer is then separated into dies using the cuts.

According to yet another aspect of the present invention, a beam delivery system comprises a beam expanding telescope for receiving a raw laser beam from a laser and for producing an expanded beam. The beam delivery system further comprises at least one variable anamorphic lens system comprising a cylindrical plano-concave lens and a cylindrical plano-convex lens for receiving the expanded beam and for producing a modified beam collimated in one principal meridian and converging in another principal meridian. The beam delivery system further comprises a beam focusing lens for receiving the modified beam and focusing the modified beam such that the focused beam has two separate focal points. One of the focal points is shorter than a nominal focal length of the beam focusing lens and the other of the focal points is formed generally at the nominal focal length of the beam focusing lens.

According to yet another aspect of the present invention, a method is provided for scribing a sapphire substrate having a layer of GaN. This method comprises directing pulses of laser energy forming an astigmatic focal beam spot at a surface of the GaN on the sapphire substrate using a solid state laser and causing the pulses to impact the sapphire substrate in a scribe pattern to cut scribe lines in the sapphire substrate. The pulses couple laser energy into the GaN layer to induce ablation of the sapphire substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be better understood by reading the following detailed description, taken together with the drawings wherein:

FIG. 1 is a schematic diagram of a beam delivery system (BDS) with astigmatic focal point optics, according to one embodiment of the present invention.

FIG. 2 is a schematic diagram of the BDS shown in FIG. 1 illustrating the sequential modification of the laser beam from the laser to the target.

FIG. 3 is a cross-sectional view of a beam, illustrating the formation of two focal points separately in each principal meridian.

FIG. 4 is a cross-sectional view of a beam focusing lens in the BDS shown in FIG. 1, illustrating the ‘y component’ of the highly compressed beam passing through the beam focusing lens.

FIG. 5 is a cross-sectional view of a beam focusing lens in the BDS shown in FIG. 1, illustrating the ‘x component’ of the highly compressed beam passing through the beam focusing lens.

FIG. 6 is a cross-sectional view of the BDS shown in FIG. 1, illustrating the formation of two separated focal points in one principal meridian.

FIG. 7 is a cross-sectional view of the BDS shown in FIG. 1, illustrating the formation of two separated focal points in the other principal meridian.

FIGS. 8 and 9 are cross-sectional views of the BDS shown in FIG. 1, illustrating the flexibility of adjusting processing parameters in the BDS.

FIG. 10 is a photograph showing a top view of one example of a sapphire-based LED wafer, scribed with the variable astigmatic focal beam spot from the BDS using a 266 nm DPSS laser.

FIG. 11 is a photograph showing a cross-sectional view of one example of a sapphire-based LED wafer, scribed with the variable astigmatic focal beam spot.

FIG. 12 is a photograph showing a top view of one example of a silicon wafer, scribed with the variable astigmatic focal beam spot from the BDS using a 266 nm DPSS laser.

FIG. 13 is a photograph showing a top view of one example of a GaP wafer, scribed with the variable astigmatic focal beam spot from the BDS using a 266 nm DPSS laser.

FIG. 14 is a photograph showing a top view of one example of a molybdenum film, scribed with the variable astigmatic focal beam spot from the BDS using a 266 nm DPSS laser.

FIG. 15 is a photograph showing one example of laser scribed lines without protective coating.

FIG. 16 is a photograph showing one example of laser scribed lines with protective coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview of BDS

Referring to FIG. 1, one embodiment of an anamorphic beam delivery system (BDS) 10 is described in detail. The anamorphic BDS 10 generates an astigmatic focal beam spot that can be used to cut or machine a substrate made of various types of materials. In one exemplary application, the preferred embodiment of the BDS 10 improves the productivity of LED die separation by forming a highly-resolved adjustable astigmatic focal beam spot, which maximizes scribing speed and minimizes consumption of scribing-related real estate on a wafer. The BDS 10 can also be used in other scribing or cutting applications.

In the embodiment shown, a solid-state laser 12, preferably diode pumped, generates a laser beam in the UV range, preferably the third harmonic 355 nm or the fourth harmonic 266 nm. The raw laser beam is usually in TEM₀₀ mode with Gaussian distribution and is enlarged by a beam-expanding telescope (BET) 14. The exemplary embodiment of the BET 14 is composed of the spherical plano-concave lens 16 and spherical plano-convex lens 18. Magnification of the BET 14 is determined by the focal lengths of each lens, generally described by M=(|f_(sx)|/|f_(sv)|), where M is magnification, f_(sx) is a focal length of the spherical plano-convex lens 18 and f_(sv) is a focal length of the spherical plano-concave lens 16. To effect collimated beam expansion, the distance between the spherical plano-concave lens 16 and the spherical plano-convex lens 18 is determined by a general equation, D_(c)=f_(sx)+f_(sv), where D_(c) is a collimation distance. Combinations of f_(sx) and f_(sv) can be used to satisfy designed values of the magnification M and the collimation distance D_(c). The range of M can be about 2× to 20×, and is preferably 2.5× in the exemplary BDS 10. Based on this preferred magnification of 2.5×, a combination of f_(sx)=250 mm and f_(sv)=100 mm with D_(c)=150 mm is preferably used in this BDS 10.

In the preferred embodiment, the expanded beam is reflected by the 100% mirror 20 a and then directed to the beam shaping iris 22. The beam shaping iris 22 symmetrically crops out the low intensity edges of the beam in a Gaussian profile, leaving a high intensity portion passing through the iris 22. The beam is then directed to the center of a variable anamorphic lens system 24.

The exemplary variable anamorphic lens system 24 is composed of a cylindrical plano-concave lens 26 and a cylindrical plano-convex lens 28. The constituents of the variable anamorphic lens system 24 preferably satisfy a condition, |f_(cx)|=|f_(cv)| where f_(cx) is a focal length of the cylindrical plano-convex lens 28 and f_(cv) is a focal length of the cylindrical plano-concave lens 26. In the variable anamorphic lens system 24, the incident beam is asymmetrically modified in one of the two principal meridians, which appears in the horizontal direction in FIG. 1. In the anamorphic lens system 24, when D<D_(c), where D is a distance between a cylindrical plano-concave lens 26 and a cylindrical plano-convex lens 28 and D_(c) is a collimation distance, a parallel incident beam is diverging after the anamorphic lens system 24. In contrast, when D>D_(c), a parallel incident beam is converging after the anamorphic lens system 24. In the preferred embodiment of the anamorphic lens system 24 shown in FIG. 1, the collimation distance is D_(c)=f_(cx)+f_(cv)=0, because |f_(cx)|=|f_(cv)| and f_(cx) has a positive value and f_(cv) a negative value and D≧D_(c), Accordingly, when D>0, the collimated incident beam is converging after the anamorphic lens system 24.

The degree of convergence or combined focal length (f_(as)) of the anamorphic system 24 is governed by the distance D, and it is generally expressed by the two lens principle: f_(as)=f_(cx)f_(cv)/(f_(cx)+f_(cv)−D). Namely, the larger the distance D, the shorter the focal length f_(as). When the distance D increases, the degree of convergence increases in only one principal meridian of the collimated incident beam. One principal meridian of the incident beam loses its collimation and converges after the variable anamorphic lens system 24; however the other principal meridian is not affected and keeps its beam collimation. Consequently, the size of the beam after the variable anamorphic lens system 24 is changed in only one principal meridian by adjusting the distance between the two lenses in the anamorphic system 24. Thus, the anamorphic BDS 10 deliberately introduces astigmatism to produce focal points separated in two principal meridians, i.e. vertical and horizontal. Although a series of anamorphic lenses in different focal lengths or convergences is preferred to provide a variable astigmatic beam spot, the variable anamorphic lens system can be replaced by a single anamorphic lens for a fixed convergence.

After the variable anamorphic lens system 24, the beam is reflected by another 100% mirror 20 b, and then directed to the center of a beam focusing lens 30. The exemplary beam focusing lens 30 is an aberration corrected spherical multi-element lens having a focal length range between about +20 mm to +100 mm. In one embodiment of the BDS 10, an edge-contact doublet with +50 mm focal length is used. After the beam focusing lens 30, one of the astigmatic focal points is sharply focused on a substrate 32, such as a semiconductor wafer. In one preferred embodiment, the substrate 32 is translated by computer controlled x-y motion stages 34 for scribing. In semiconductor scribing applications where the semiconductor wafer contains square or rectangular dies, the semiconductor wafer can be rotated 90 degrees by a rotary stage 36 for scribing in both the x direction and the y direction.

The preferred combination of the BET 14 and the multi-element beam focusing lens 30 yields a highly-resolved and adjustable astigmatic focal beam spot with minimal aberration and a minimized beam waist diameter. In general, a minimum beam waist diameter (w_(o)) of a Gaussian beam can be expressed by: w_(o)=λf/πw_(i) where λ is a wavelength of an incident laser beam, f is a focal length of a beam focusing lens, π is the circular constant, and w_(i) is a diameter of the incident beam. In a given beam focusing lens 30, the minimum beam waist diameter (w_(o)) or a size focused spot is inversely proportional to the incident beam diameter (w_(i)). In the exemplary embodiment of the present invention, the BET 14 anamorphically increases the incident beam diameter (w_(i)) which is focused by the multi-element beam focusing lens 30, resulting in a minimized beam waist diameter and yielding a highly-resolved focal beam spot. This provides a sharply focused scribing beam spot capable of providing about 5 μm or less scribing kerf width on a semiconductor wafer. Consequently, the minimized scribing kerf width significantly reduces consumption of real estate on a wafer by scribing, which allows more dies on a wafer and improves productivity.

The combination of the variable anamorphic lens system 24 and the high resolution beam focusing lens 30 results in two separate focal points in each principal meridian of the incident beam. The flexibility of changing beam convergence from the variable anamorphic lens system 24 provides an instant modification of a laser energy density on a target semiconductor wafer. Since the optimum laser energy density is determined by light absorption properties of the particular target semiconductor wafer, the variable anamorphic lens system 24 can provide an instant adaptation to the optimum processing condition determined by various types of semiconductor wafers.

Although one exemplary embodiment of the anamorphic BDS 10 is shown and described, other embodiments are contemplated and within the scope of the present invention. In particular, the anamorphic BDS 10 can use different components to create the astigmatic focal beam spot or the anamorphic BDS 10 can include additional components to provide further modification of the beam.

In one alternative embodiment, a bi-prism 38 or a set of bi-prisms can be inserted between the anamorphic lens system 24 and the BET 14. The bi-prism equally divides the expanded and collimated beam from the BET 14, then crosses the two divided beams over to produce an inversion of half Gaussian profile. When a set of bi-prisms is used, the distance between the two divided beams can be adjusted by changing the distance between the set of bi-prisms. In other words, the bi-prism 38 divides the Gaussian beam by half circles and inverts the two divided half circles. A superimposition of these two circles creates superimposition of the edges of Gaussian profiles in weak intensity. This inversion of a Gaussian profile and intensity redistribution creates a homogeneous beam profile and eliminates certain drawbacks of a Gaussian intensity profile.

In another embodiment, the BDS 10 can include an array of anamorphic lens systems 24 used to create small segments of separated astigmatic ‘beamlets’, similar to a dotted line. The astigmatic beamlets allow an effective escape of laser-induced plasma, which positively alters scribing results. The distance between the lenses in the array of anamorphic lens systems controls the length of each segment of the beamlets. The distance among the segments of the beamlets can be controlled by introducing a cyclindrical plano-convex lens in front of the array of anamorphic lens systems.

Generation of Variable Astigmatic Focal Beam Spot

Referring to FIG. 2, one method of forming a variable astigmatic focal beam spot is described in greater detail. The profile of raw beam 50 from the laser generally has about 0.5 mm to 3 mm of diameter in a Gaussian distribution. The raw beam 50 is expanded by the BET 14 and the expanded beam 52 is about 2.5 times larger in diameter. The expanded beam 52 is passed through the beam shaping iris 22 for edge cropping and the expanded and edge-cropped beam 54 is directed to the center of the anamorphic lens system 24. The anamorphic lens system 24 modifies the expanded and edge-cropped beam 54 in only one principle meridian, resulting in a slightly compressed beam shape 56. As the slightly compressed laser beam 56 travels towards the beam focusing lens 30, the degree of astigmatism is increased in the beam shape since the variable anamorphic lens system 24 makes the beam converge in only one principal meridian. Subsequently, the highly compressed beam 57 passes through the beam focusing lens 30 to form the astigmatic focal beam spot 58. Since the highly compressed beam 57 has converging beam characteristics in one principal meridian and collimated beam characteristics in the other, focal points are formed separately in each principal meridian after the beam focusing lens 30. Although this method of forming the astigmatic focal beam spot 58 is described in the context of the exemplary BDS 10, this is not a limitation on the method.

The three-dimensional diagram in FIG. 3 illustrates in greater detail the formation of the two focal points separately in each principal meridian when the highly compressed beam 57 passes through the beam focusing lens (not shown). Since the highly compressed beam 57 in one principal meridian (hereinafter the ‘y component’) has converging characteristics, the y component exhibits the short distance focal point 60. In contrast, since the other meridian (hereinafter the ‘x component’) has collimating beam characteristics, the x component exhibits the long distance focal point 62. Combination of the x and y components results in the astigmatic beam spot 58.

FIG. 4 shows the y component of the highly compressed beam 57, which passes through the beam focusing lens 30 and results in the focal point 60. After the focal point 60, the beam diverges and creates the astigmatic side of the beam spot 58.

FIG. 5 shows the x component of the highly compressed beam 57, which passes through the beam focusing lens 30 and results in the focal point 62. The collimated x component of the highly compressed beam 57 is sharply focused at the focal point 62, which creates the sharply focused side of the astigmatic beam spot 58.

FIGS. 6 and 7 illustrate further the formation of two separated focal points 60, 62 in each principal meridian. The schematic beam tracings in FIGS. 6 and 7 include two-dimensional layouts of the BDS 10 shown in FIG. 1 excluding the 100% mirrors 20 a, 20 b and the beam shaping iris 22 for simplicity. In FIG. 6, the raw beam from the solid-state laser 12 is expanded by the BET 14 and then collimated. The variable anamorphic lens system 24 modifies the collimated beam in this principle meridian, resulting in convergence of the beam. The converging beam is focused by the beam focusing lens 30. Due to its convergence from the variable anamorphic lens system 24, the beam forms the focal point 60, shorter than the nominal focal length of the beam focusing lens 30. The beam tracing in FIG. 6 is analogous to the view of the y component in FIG. 4.

In contrast, in FIG. 7, the expanded and collimated beam from BET 14 is not affected by the variable anamorphic lens system 24 in this principal meridian. The collimation of the beam can be maintained in this meridian after the variable anamorphic lens system 24. After passing though the beam focusing lens 30, the collimated beam is focused at the focal point 62, which is formed at a nominal focal length of the beam focusing lens 30. The beam tracing in FIG. 7 is analogous to the view of the x component in FIG. 5. In FIG. 7, the BET 14 increases the incident beam diameter, which is focused by the multi-element beam focusing lens 30, resulting in minimized a beam waist diameter and yielding a highly-resolved focal beam spot. As a result, the target substrate 32 (e.g., a semiconductor wafer) receives a wide and defocused astigmatic beam in one principal meridian and a narrow and sharply focused beam in the other principal meridian.

As illustrated in FIG. 3, the combination of these two separated focal points 60, 62 generates an astigmatic beam spot having one side with a defocused and compressed circumference and the other side with a sharply focused and short circumference.

Scribing Applications Using an Astigmatic Focal Beam Spot

To scribe a substrate, the astigmatic focal beam spot is directed at the substrate and applied with a set of parameters (e.g., wavelength, energy density, pulse repetition rate, beam size) depending upon the material being scribed. According to one method, the astigmatic focal beam spot can be used for scribing semiconductor wafers, for example, in wafer separation or dicing applications. In this method, the wafer can be moved or translated in at least one cutting direction under the focused laser beam to create one or more laser scribing cuts. To cut dies from a semiconductor wafer, a plurality of scribing cuts can be created by moving the wafer in an x direction and then by moving the wafer in a y direction after rotating the wafer 90 degrees. When scribing in the x and y directions, the astigmatic beam spot is generally insensitive to polarization factors because the wafer is rotated to provide the cuts in the x and y directions. After the scribing cuts are made, the semiconductor wafer can be separated along the scribing cuts to form the dies using techniques known to those skilled in the art.

The astigmatic beam spot provides an advantage in scribing applications by enabling faster scribing speeds. The scribing speed can be denoted by S=(l_(b)·r_(p))/n_(d), where S is the scribing speed (mm/sec), l_(b) is the length of the focused scribing beam (mm), r_(p) is pulse repetition rate (pulse/sec) and n_(d) is the number of pulses required to achieve optimum scribing cut depth. The pulse repetition rate r_(p) depends on the type of laser that is used. Solid state lasers with a few pulses per second to over 10⁵ pulses per second are commercially available. The number of pulses n_(d) is a material processing parameter, which is determined by material properties of the target wafer and a desired cut depth. Given the pulse repetition rate r_(p) and the number of pulses n_(d), the beam length l_(b) is a controlling factor to determine the speed of the cut. The focused astigmatic beam spot formed according to the method described above increases the beam length l_(b) resulting in higher scribing speeds.

The preferred BDS 14 also provides greater flexibility to adjust processing parameters for achieving an optimum condition. In laser material processing, for example, processing parameters should preferably be adjusted for optimum conditions based on material properties of a target. The overflow of laser energy density can result in detrimental thermal damage to the target, and the lack of laser energy density can cause improper ablation or other undesired results. FIGS. 8 and 9 show the flexibility of adjusting processing parameters of the BDS in this invention.

In FIG. 8, the lenses 26, 28 of the variable anamorphic lens system 24 are placed close together, which results in low convergence of the collimated incident beam. This low convergence forms the focal point 60 at a relatively further distance from the beam focusing lens 30. Consequently, the length of the beam spot 58 is relatively shorter.

In contrast, in FIG. 9, the lenses 26, 28 of the variable anamorphic lens system 24 are placed further apart, which results in high convergence of the collimated incident beam. This increased convergence introduces astigmatism and forms the focal point 60 at a relatively shorter distance from the beam focusing lens 30. Consequently, the length of the beam spot 58 is relatively longer.

In one scribing example, the astigmatic focal beam spot can be used to scribe a sapphire substrate used for blue LEDs. Optimum processing of a sapphire substrate for blue LEDs generally requires an energy density of about 10 J/cm². Since blue LED wafers are generally designed to have about a 50 μm gap among the individual die for separation, the optimum laser beam size is preferably less than about 20 μm for laser scribing. When a currently-available commercial laser with 3 Watts on target output at 50 kHz pulse repetition is used, the conventional beam focusing at a 15 μm diameter results in laser energy density of 34 J/cm². In a system with conventional beam spot focusing, the energy density on target has to be adjusted by reducing the power output of the laser for optimum processing to avoid an overflow. Thus, the laser power output cannot be fully utilized to maximize the scribing speed or productivity.

In contrast, the preferred embodiment of the BDS 10 can adjust the size of the compressed beam spot to maintain the optimum laser energy density for 10 J/cm² without reducing the power output from the laser. The size of the astigmatic beam can be adjusted to have about 150 μm in the astigmatic axis and about 5 μm in the focused axis. Since the astigmatic axis is lined up in the scribing translation direction, this increase in beam length proportionally increases the scribing speed as discussed above. In this example, the astigmatic beam spot can provide processing speeds that are about 10 times faster than that of conventional beam focusing.

In another scribing example, the astigmatic focal beam spot can be used to scribe a sapphire substrate by coupling with one or more GaN layers on the sapphire substrate (e.g., about 4˜7 μm over the sapphire substrate) instead of coupling directly with sapphire. The lower bandgap of GaN provides more efficient coupling with the incident laser beam, requiring only about 5 J/cm² for the laser energy density. Once the laser beam couples with GaN, the ablation through the sapphire substrate is much easier than direct coupling with the sapphire. Accordingly, the size of the astigmatic beam can be adjusted to have about 300 μm in the astigmatic axis and about 5 μm in the focused axis. Thus, the processing speed can be 20 times faster than the conventional far field imaging or spot focusing techniques.

The minimized spot size in the focused axis also significantly reduces the scribing kerf width, which subsequently reduces consumption of a wafer real estate. Furthermore, by reducing total removed material volume, the narrow scribing cuts reduce collateral material damage and ablation-generated debris. FIG. 10 shows an example of a sapphire based LED wafer, scribed with the astigmatic focal beam spot from the BDS 10 using a 266 nm DPSS laser with on target power of about 1.8 Watt at 50 kHz. The size of the astigmatic beam was adjusted to have about 180 μm in the astigmatic axis and about 5 μm in the focused axis. Viewed from the top, FIG. 10 shows a cut width of about 5 μm. Based on 30 μm deep scribing, the BDS 10 is capable of scribing speeds of greater than 50 mm/sec. The laser cut forms a sharp V-shaped groove, as shown in FIG. 11, which facilitates well controlled fracturing after the scribing. The variable astigmatic focal beam spot from the adjustable BDS 10 utilizes the maximum power output from the laser, which directly increases the processing speeds. Thus, front side scribing can be used to decrease the street width and increase fracture yield, thereby increasing usable die per wafer.

The astigmatic focal beam spot can also be used advantageously to scribe other types of semiconductor wafers. The astigmatic focal beam spot readily adjusts its laser energy density for an optimum value, based on the target material absorption properties, such as bandgap energy and surface roughness. FIG. 12 shows an example of a silicon wafer, scribed with the astigmatic focal beam spot from the BDS 10 using a 266 nm DPSS laser with on target power of about 1.8 Watt at 50 kHz. The size of the astigmatic beam was adjusted to have about 170 μm in the astigmatic axis and about 5 μm in the focused axis. This resulted in 75 μm deep scribing with a speed at about 40 mm/sec.

A comparable result is shown in FIG. 13 for a GaP wafer with the same laser and on target power. The size of the astigmatic beam was adjusted to have about 300 μm in the astigmatic axis and 5 μm in the focused axis. This resulted in 65 μm deep scribing with a speed at about 100 mm/sec. Similar results were found in other compound semiconductor wafers such as GaAs and Ge. Other substrates that can be scribed include, but are not limited to, InP, Alumina, glass and polymers.

The astigmatic focal beam spot can also be used advantageously to scribe or machine metal films such as copper or molybdenum. Due to high thermal conductivity, laser cutting of metal films using conventional techniques has shown extensive heat affected zones along the wake of the laser cut. With the application of the astigmatic focal beam spot, the 5 μm beam width in the focused axis significantly reduces a laser cutting kerf width, which subsequently reduces heat affected zones, collateral material damage and ablation-generated debris. As an example, FIG. 14 shows narrow and shapely resolved cut lines on molybdenum. The size of the astigmatic beam was adjusted to have about 200 μm in the astigmatic axis and about 5 μm in the focused axis. This resulted in 50 μm deep scribing with a speed at about 20 mm/sec, using 266 nm DPSS laser with on target power of about 2.5 Watt at 25 kHz. Other types of metal can also be cut.

Although the examples show lines scribed in a substrate, the astigmatic focal beam spot can also be used to scribe other shapes or to perform other types of machining or cutting applications. Operating parameters other than those given in the above examples are also contemplated for scribing LED wafers. For example, a 355 nm DPSS laser can also be used for LED scribing applications, although the 266 nm DPSS laser is preferable to minimize the thermal damage that can cause lower light output of the LED.

According to another scribing method, surface protection can be provided on the substrate by using a water soluble protective coating. The preferred composition of the protective coating comprises at least one surfactant in a water-soluble liquid glycerin and can be any kind of generic liquid detergent that satisfies this compositional requirement. The surfactant in the liquid glycerin forms a thin protective layer due to its high wetability. After the thin film layer is dried off, the glycerin effectively endures heat from the laser induced plasma, while preventing laser generated debris from adhering on the surface. The thin film of liquid detergent is easily removed by cleaning with pressurized water. FIG. 14 illustrates the laser scribing on a LED wafer without the surface protection, showing a significant amount of debris accumulated along the laser cut. In contrast, FIG. 15 illustrates the laser scribing on a LED with protective coating using a liquid detergent, which prevented the laser induced debris on the LED surface.

Accordingly, the preferred embodiment of the present invention provides advantages over conventional systems using patterned laser projection and conventional systems using far field imaging. Unlike simple far field imaging, the present invention provides greater flexibility for modifying the laser beam by using the anamorphic BDS to produce the astigmatic focal beam spot. Unlike conventional patterned laser projection, the anamorphic BDS delivers substantially the entire beam from a laser resonator to a target, thus maintaining very high beam utilization. The formation of the astigmatic focal beam spot also allows the laser beam to have excellent characteristics in both the optimum intensity and the beam waist diameter. In particular, the preferred embodiment of the variable anamorphic lens system enables an adjustable uniplanar compression of a laser beam, which results in a variable focal beam spot for prompt adjustments of the optimum laser intensity. By proper modification of beam spot and by maximized utilization of a raw beam, the formation of the astigmatic focal beam spot results in numerous advantages on separation of various semiconductor wafers, including fast scribing speeds, narrow scribing kerf width, reduced laser debris, and reduced collateral damage.

While the principles of the invention have been described herein, it is to be understood by those skilled in the art tat this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims. 

1. A method for forming an astigmatic focal beam spot to ablate and cut a substrate, said method comprising: determining a target energy density for said substrate; generating a raw laser beam; expanding said raw laser beam; modifying said expanded beam such that said modified beam is collimated in one principal meridian and converging in another principal meridian; focusing said modified beam to produce an astigmatic focal beam spot on said substrate, said astigmatic focal beam spot having an elongated shape with a focused axis having a first focal point and an astigmatic axis having a second focal point separate from said first focal point, said astigmatic focal beam spot having a length along said astigmatic axis and a width along said focused axis, the width being less than the length, wherein said astigmatic focal beam spot is modified and focused to provide an energy density on said substrate at the target energy density for said substrate; and moving said substrate in a cutting direction along said length of said astigmatic focal beam spot such that said astigmatic focal beam spot causes ablation of said substrate to obtain at least a partial cut in said substrate.
 2. The method of claim 1 wherein said raw beam is generated using a solid state laser.
 3. The method of claim 2 wherein said raw beam is generated in a UV range less than about 400 nm.
 4. The method of claim 3 wherein said raw beam is generated with a pulse duration less than about 40 ns.
 5. The method of claim 1 wherein expanding said raw beam includes passing said raw beam through a beam expanding telescope.
 6. The method of claim 1 wherein modifying said expanded beam includes passing said expanded beam through an anamorphic lens system comprising a cylindrical plano-concave lens and a cylindrical plano-convex lens.
 7. The method of claim 1 further comprising varying the convergence of said modified beam to adjust the energy density provided by said astigmatic focal beam spot.
 8. The method of claim 1 wherein modifying said expanded beam includes passing said expanded beam through a single anamorphic lens to provide a fixed convergence.
 9. The method of claim 1 further comprising symmetrically cropping low intensity edges of said expanded beam.
 10. The method of claim 1 wherein focusing said modified beam comprises passing said modified beam through a beam focusing lens, wherein said second focal point is shorter than a nominal focal length of said beam focusing lens and said first focal point is formed generally at said nominal focal length of said beam focusing lens.
 11. The method of claim 1 wherein said substrate includes sapphire.
 12. The method of claim 11 wherein said substrate includes a GaN layer on said sapphire, and wherein said astigmatic focal beam spot is directed at a surface of said GaN layer such that laser energy is coupled into said GaN layer to cause ablation of said sapphire.
 13. The method of claim 1 wherein said substrate is part of a semiconductor wafer including a device layer on said substrate, and wherein moving said substrate includes moving said semiconductor wafer along the length of said astigmatic focal beam spot to form at least one scribe line in said semiconductor wafer.
 14. The method of claim 1 wherein said substrate is made of a material selected from the group consisting of metal, GaAs, silicon, GaP, InP, Ge, alumina, glass and polymers.
 15. The method of claim 1 wherein said astigmatic focal beam spot has a width of less than about 20 μm.
 16. The method of claim 15 wherein said astigmatic focal beam spot has a width of about 5 μm.
 17. The method of claim 1 wherein modifying said expanded beam includes creating a plurality of separated astigmatic beamlets.
 18. The method of claim 17 wherein modifying said expanded beam includes controlling at least one of a length of said beamlets and a distance between said beamlets.
 19. The method of claim 13 wherein moving said substrate along the length of said astigmatic focal beam spot includes moving said semiconductor wafer to form a plurality of scribe lines in said semiconductor wafer.
 20. The method of claim 19 further comprising separating said semiconductor wafer into dies using said plurality of scribe lines.
 21. The method of claim 1 wherein moving said substrate in a cutting direction comprises: moving said substrate in a cutting direction along the length of said astigmatic focal beam spot such that at least one said at least a partial cut is formed in an x direction on said substrate; rotating said substrate about 90 degrees; and moving said substrate in a cutting direction along the length of said astigmatic focal beam spot such that at least one said at least a partial cut is formed in a y direction on said substrate.
 22. The method of claim 6 further comprising varying an aspect ratio of said astigmatic focal beam spot to adjust the energy density provided by said astigmatic focal beam spot by varying a spacing between said cylindrical plano-concave lens and said cylindrical plano-convex lens.
 23. The method of claim 12 wherein moving said substrate includes moving said sapphire substrate along a the length of said astigmatic focal beam spot to form at least one scribe line in said sapphire substrate.
 24. The method of claim 1 wherein said substrate includes a metal film made of a metal selected from the group consisting of molybdenum and copper.
 25. The method of claim 1 further comprising: prior to causing ablation, applying a water soluble protective coating to said substrate, said protective coating including at least one surfactant in a water-soluble liquid glycerin. 